This form of vacuum gauge was invented in 1874 By H. G. McLeod
to measure pressures of the order of 10E-2 to 10E-7 Torr. A slug of mercury
moving in a tube is used to isolate a volume of gas at the pressure to
be measured. The gas in the volume is then compressed by a known amount,
and the final pressure is obtained with a manometer. Boyle's law is then
used to find the initial pressure from the final pressure, and the initial
and final volumes. This is a relatively slow and laborious process, and
a liquid nitrogen cold trap must be used to prevent the vapor pressure
of mercury from perturbing the pressure measurements. However, the readings
of the McLeod gauge are absolute, and it is used to calibrate other gauges,
such as ionization gauges.

The example at the left was located on the wall of Prof.
Wolfgang Christian's office at the Davidson College physics department
when the picture was taken in April 2000.

At the right is a McLeod gauge at Kenyon
College that has not been used since ca. 1950.